In the related art, in the case of producing an external screw having a thread portion of any one of a right screw and a left screw through form-rolling, a screw thread or a thread groove is generally formed by pressing a screw material that is a rod-like body in a metal columnar shape, which is also referred to as a blank, through a plurality of rigid plates or a plurality of cylindrical bodies each having, on its surface, a lead portion of a multi-thread that is formed to have a cross-section in an intended shape, for example, a substantially triangular shape, and has a lead angle to be nearly in parallel to each other, by relatively displacing the screw material and the rigid plates or the rigid cylindrical bodies, and by plastically deforming a surface of the screw material.
Also, a compound screw in a form of an externally-threaded body is known and the compound screw has a right-hand thread portion and a left-hand thread portion on the same region in an axial direction of a thread portion in the externally-threaded body. Although an attempt to produce such the compound screw through form-rolling has been made, a form-rolling method that enables a mass production in a reasonable and precise manner has not been currently realized yet.
A technology disclosed in Reference 1 may refer to a technology for form-rolling of a compound screw. According thereto, employing a screw material processed in a noncircular shape may be construed as being preferable. However, to use a noncircular object as the screw material, processing the screw material in advance in a noncircular shape is essential. Also, in practice, although a noncircular screw material is used, the screw material may slip in a die or it may be difficult to set an initial location of the screw material with respect to a form-rolling die. Further, an intended type, that is, an externally-threaded body in a compound screw structure was unable to be produced.
Also, Reference 1 relates to a producing method of a die, and discloses a method of producing a form-rolling die that may produce a master die having an external appearance of a compound screw that is a final product, and may roll the master die while pressing the master die against a die plate. However, when producing the form-rolling die using the above method, a curved surface in a continuous waveform is unintentionally formed on the surface of the die. Accordingly, when performing form-rolling using the form-rolling die, there were issues, such as that an axial shape is not provided in a columnar shape that is a true circular shape and instead, is unstable, and a production of a threaded body having a root diameter with which screwing an internal screw is impossible or difficult.
Also, in technologies disclosed in Reference 2 and Reference 3, when performing form-rolling using a form-rolling die disclosed therein, form-rolling of a compound screw having a right-hand thread portion and a left-hand thread portion on the same region in an axial direction of a thread portion may be performed. However, a right screw and a left screw of the compound screw have different pitches. Form-rolling of a compound screw having a right-hand thread portion and a left-hand thread portion having the same pitch may not be performed.
In particular, according to the technology disclosed in Reference 3, a half of issues of defective products occurring due to rolling defects such as slips occurring between a screw material and a form-rolling die during form-rolling, which is an issue in Reference 1 or Reference 2, may be solved. However, there were issues, such as that a minor metal piece, called shavings, generated when the screw material is cut by the surface of the form-rolling die is attached with being stuck in the uneven surface of the form-rolling die and thus, form-rolling of a highly precise compound screw cannot be consecutively performed.